The present invention relates to sports rackets such as tennis rackets, and particularly to an improved inflatable suspension tube for use in a gas-suspension racket of the type having a stringed insert located within a surrounding outer frame and mechanically coupled resiliently with the outer frame by the presence of such a suspension tube within opposing channels defined on the interior of the outer frame and the exterior of the insert.
Maynard U.S. Pat. No. 4,357,012 discloses a sports racket having a removable stringed insert located within a surrounding outer frame and connected with the outer frame by an inflatable tube located between the outer frame and the inner frame and extending within opposing channels surrounding the exterior of the inner insert and the interior of the outer frame. Such a racket is intended to provide better power in striking a ball, and to avoid transmitting the shock of striking the ball to the player's elbow joint. For a tennis racket of this type to be completely satisfactory, it must be able to be manufactured in an embodiment which is not too heavy and in which the balance of the complete racket is such that the racket is not excessively head-heavy. If the racket is too head-heavy, a player will not be able to swing it quickly enough or control it precisely enough to strike a ball as desired.
One of the objectives of a gas-suspension racket is to provide resiliency to cushion the handle of the racket from the shock of the strings striking a ball, but without having to loosen the tension of the strings of the racket to provide such cushioning. The suspension of the insert within the outer frame by a resilient gas-filled suspension tube achieves this objective, but only if a certain amount of clearance is maintained between the inwardly-facing surfaces of the outer frame and the outwardly-facing surfaces of the insert. This clearance is required in order for the insert to be able to move predictably in response to striking a ball, with relative movement being opposed, and the original position of the insert being restored, by the elasticity of the suspension tube and the gas contained within it.
The heads of most sports rackets are oval, not circular. As a result, an inflatable suspension tube having uniform dimensions along its entire circumference exerts a greater force toward the longer opposite sides of the head of a gas-suspension racket, where the radius of curvature is usually greater, than toward the top and bottom of the racket head, because of the greater length over which the force tending to expand the suspension tube is exerted in opposite directions between the insert and the outer frame of the racket head. If the racket head frame and the frame of the insert are of uniform dimensions about their entire circumferences, there will be a resulting tendency of the sides of the outer frame of the racket head to be displaced outwardly and for the top to move slightly toward the throat of the racket, reducing clearance between the outer frame and the insert at the top and throat. This force exerted by the suspension tube may also effectively loosen the horizontal strings by distorting the frame of the insert.
While it might be possible to design a racket head having a combination of frame and insert in which distortion of the frame with a particular inflation pressure in the suspension tube would result in the desired clearances, the result is likely to be difficulty in removal and replacement of the insert. It is also generally not practical to design such a racket head, because of the different amounts of pressure which players might desire to use under different playing conditions. Such different pressures in the suspension tube could well result in unacceptable variation of the clearances between the racket head's outer frame and the insert.
Loss of clearance between the outer frame and the insert may cause interference between the frame and the insert at times during play, which makes the response of the racket somewhat unpredictable. Interference between the racket head's outer frame and the insert also eliminates the desirable ability of the suspension tube to absorb and restore energy as the ball strikes the strings and rebounds away from the racket. Such interference also results in shock being transmitted from the racket strings to the handle of the racket, defeating one of the main objectives of the gas-suspension racket.
While use of sturdy enough construction of a racket head outer frame and insert frame, or use of a low inflation pressure of the suspension tube, might result in avoiding problems of mechanical interference between the outer frame and the frame of the insert, it is usually desirable to inflate the suspension tube to a relatively high pressure, in order to retain the insert securely within the outer frame and to provide a desired amount of mechanical coupling between the racket handle and the insert. These objectives must be achieved, however, without the head of the racket becoming unacceptably heavy as a result of the structure used.
What is needed, therefore, is an improved suspension tube structure for use in a game racket of the type described, which enables the suspension tube to be pressurized to a high enough pressure to couple the insert firmly to the outer frame, without distorting the outer frame of the racket head, without an unacceptable racket head weight, without loss of resiliency of the suspension tube, and while retaining ample flexibility of the suspension tube to permit replacement of the insert within the outer frame of the racket head by deflating the suspension tube.